Under a background of the growing environmental problems in recent years, hydroelectric power generation, wind power generation and photovoltaic power generation are getting into the limelight as clean energy. Out of these, the spread of photovoltaic power generation has been making remarkable progress in recent years as the performance of solar cell modules in power generation efficiency and other respects has been making marked improvements while on the other hand their prices have been falling and the national and local governments have been promoting the business of introducing photovoltaic power generation systems for household use. Under a backdrop of such expansion of the solar cell market, there is a growing demand for higher productivity in the manufacture of solar cell modules.
A solar cell module is generally a package formed by protecting a solar cell element comprising such materials as silicon, gallium-arsenic and copper-indium-selenium with a top transparent protective material and a bottom protective substrate material, with the solar cell element and the protective materials fixed by use of an encapsulating material. Generally, the encapsulating material for the solar cell element in a solar cell module is formed by extruding a sheet from an ethylene-vinyl acetate copolymer to which an organic peroxide and silane coupling agent have been compounded. The encapsulating material in the form of sheet thus obtained is used to encapsulate the solar cell element to produce a solar cell module. In this method, it is necessary at the stage of the production of an encapsulating material in the form of sheet to form the encapsulating material at such low temperature that the organic peroxide will not be decomposed, and therefore it is impossible to increase the speed of extrusion. In this method, it is also necessary at the stage of encapsulating the solar cell element to go through a two-step crosslinking and bonding process which comprises the step of carrying out preliminary bonding on a laminator in several minutes to several tens of minutes and the step of carrying out final bonding in an oven in several tens of minutes to one hour at a high temperature at which the organic peroxide is decomposed (Patent Reference 1).
As a means for improving productivity in the production of solar cell modules as described above, use of an organic peroxide showing a low decomposition temperature is conceived to increase crosslinking speed and thereby to shorten the time required for crosslinking and bonding. On the other hand, use of an organic peroxide showing a low decomposition temperature has a shortcoming that such use necessitates lower processing temperature in the sheet extrusion mentioned above to prevent the decomposition of the organic peroxide, and it causes a decline in lower productivity. To improve this shortcoming, a method for producing an encapsulating material by using a method in which a sheet of ethylene copolymer is prepared by extrusion in advance and an organic peroxide in a liquid state impregnates the resultant sheet has already been proposed (Patent Reference 2). Specifically in this proposal, an example is given in which an organic peroxide is coated onto a sheet, and the sheet is subjected to crosslinking treatment when its surface has become dry. However, with such example, there is a possibility that the organic peroxide will not be dispersed adequately in the sheet and a uniformly crosslinked sheet will not be obtained. Another shortcoming of the use of an organic peroxide showing a low decomposition temperature is that a blister phenomenon of the sheet takes place due apparently to the decomposition gas emitted as a result of the decomposition of the organic peroxide.
On the other hand, a formulation of an ethylene copolymer to which a particular organic peroxide mixture has been compounded is already been known as a compound which meets both of the requirements for the high-speed crosslinking of the ethylene copolymer and the prevention of a blister phenomenon mentioned above (Patent Reference 3). Specifically, this proposal discloses only the conventional solar cell encapsulating material manufacturing method in which a sheet is molded after an organic peroxide is compounded to an ethylene copolymer. Therefore, in this specific example, the conditions for molding an encapsulating material in the form of sheet are restricted, and it is difficult to achieve an adequate improvement in productivity.    Reference 1: Japanese Patent Publication HEI 2-40709    Reference 2: Japanese Laid-open Patent Application SHO 59-138234    Reference 3: Japanese Laid-open Patent Application HEI 11-26791